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Wiring and Installation
31.25 kbit/s, Voltage
Mode, Wire Medium
Application Guide
FOUNDATION™ fieldbus
Compliments of:
Preface
FOUNDATION Fieldbus Application Guide
31.25 kbit/s Wiring and Installation
AG-140 Revision 1.0
This application guide has been prepared to aid understanding of the application considerations
of the FOUNDATION fieldbus.
The guide begins with a brief overview of 31.25 kbit/s wiring topologies.
The main portion of the guide provides practical guidance for the planning, design, and
installation of the 31.25 kbit/s fieldbus.
I sincerely hope that this information proves useful to you. Please contact the Fieldbus Foundation
if you need additional information about this exciting new technology.
David A. Glanzer
Quality Director
For additional information please contact:
Fieldbus Foundation
9390 Research Boulevard
Suite II-250
Austin, TX 78759
USA
Voice: 512 794 8890
Fax: 512 794 8893
Visit our World Wide Web Site:
http://www.fieldbus.org
DISCLAIMER OF WARRANTIES
This document is provided on an “as is” basis and may be subject to future additions, modifications, or corrections. The Fieldbus
Foundation hereby disclaims all warranties of any kind, express or implied, including any warranty of merchantability or fitness
for a particular purpose, for this document. In no event will the Fieldbus Foundation be responsible for any loss or damage
arising out of or resulting from any defect, error or omission in this document or from anyone’s use of or reliance on this
© 1996 Fieldbus Foundation, Austin, Texas. All rights reserved.
Table of Contents
1.0
PURPOSE . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1
2.0
SCOPE . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1
3.0
INTRODUCTION . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1
4.0
GETTING STARTED . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 2
4.1
Building the Network . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 2
4.2
Adding to the Network . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 5
4.3
Spurs . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 6
4.4
Repeaters . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 7
4.5
Mixing Cables . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 7
4.6
Shielding (Screening) . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 8
4.7
Polarity . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 10
4.8
DC Power for Two-Wire Field Devices . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 11
4.9
Intrinsic Safety . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 13
4.10 Live Wire . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 13
4.11 What Not to Connect . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 13
4.12 Connecting to Higher Speed Fieldbus Networks . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 13
4.13 What If It’s Not Wired Correctly? . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 13
5.0
6.0
FIELDBUS TOPOLOGIES . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 14
5.1
Point-to-Point Topology . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 14
5.2
Bus with Spurs Topology . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 14
5.3
Daisy-Chain . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 15
5.4
Tree Topology . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 15
5.5
Combinations of the Above . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 15
FIELDBUS COMPONENTS AND CHARACTERISTICS . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 16
6.1
Cable . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 16
6.2
Terminal Blocks . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 17
© 1996 Fieldbus Foundation, Austin, Texas. All rights reserved.
Table of Contents
6.3
Connectors (Optional) . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 17
6.4
Couplers (Optional) . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 17
6.5
Terminators . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 17
6.6
Power Supplies . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 18
6.7
Intrinsic Safety Issues . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 18
6.8
Grounding, Shields, Polarity . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 18
6.9
FOUNDATION Device Types . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 19
6.10 Special Communications Devices (Repeaters, Bridges, and Gateways) . . . . . . . . . . . . . . . . . . . . . 19
6.11 Maximum Cable Segment Lengths and Elements per Segment . . . . . . . . . . . . . . . . . . . . . . . . . . . . 20
7.0
EXAMPLE FIELDBUS APPLICATIONS . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 21
7.1
Waste Treatment Plant Example . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 21
7.2
Paper Mill Retrofit Example . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 24
8.0
QUESTIONS AND ANSWERS . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 27
9.0
DEFINITIONS . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 29
10.0 TROUBLESHOOTING . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 31
10.1 Fieldbus Equipment with Built-in Diagnostics . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 31
10.2 Reduction to Known Working State . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 31
10.3 Addressing and Polarity . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 31
10.4 Works On Bench But Not in Network . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 31
10.5 Open Trunk . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 31
10.6 Handheld Master Works At One End of Trunk But Not Other End . . . . . . . . . . . . . . . . . . . . . . . . . . 32
10.7 Working Network Suddenly Stops . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 32
10.8 Test Equipment . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 32
11.0 REFERENCES . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 33
11.1 Documents . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 33
11.2 Organizations . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 33
© 1996 Fieldbus Foundation, Austin, Texas. All rights reserved.
Introduction
1.0 PURPOSE
3.0 INTRODUCTION
This manual is intended to be used by Instrument
Engineers, Instrument Technicians, Electricians, and
Installers of 31.25 kbit/s wiring and associated
components for FOUNDATION fieldbus applications.
This Application Note is intended to provide guidelines
for wiring and installing a FOUNDATION fieldbus network.
Such a network is defined as a digital, two-way, multidrop communication link among multiple intelligent field
devices and automation systems, as defined by the
Fieldbus Foundation Specifications. A fieldbus segment
is made up of one or more segments. Each segment
typically contains several active devices, a required
power supply (for bus-powered devices), and special
terminators located at the end of each segment. For
simplicity, this document will normally discuss fieldbus
networks with just one segment. Because FOUNDATION
fieldbus is a purely digital protocol, the rules for installing
and maintaining fieldbus wiring are considerably different
from traditional 4-20 mA wiring. This document will
present some guidelines for installing a fieldbus network.
2.0 SCOPE
This manual provides application notes and advice for
wiring and installation of 31.25 kbit/s FOUNDATION fieldbus
networks. It is based on the current approved definition
of the Fieldbus Foundation Physical Layer. It is based
also on the appropriate IEC and ISA Physical Layer
Standards which are identified in Section 10.0 of this
document.
This manual does not cover the following:
• Guidelines for design of control loops using fieldbus
Section 4.0 of this document tells you how to get
started. It presents the basic concepts behind building
and adding to a fieldbus network pointing out some of
the differences between a fieldbus network and
traditional 4-20 mA current loop wiring. This section
goes on to explain spurs, repeaters, mixing different
cable types, shielding, polarity, intrinsic safety, and
connecting to other networks. Section 4.0 is
recommended for readers who have not worked with a
fieldbus network before.
• Loading or timing issues related to fieldbus
communications
• Information related to other Physical Layer options
such as:
• 1.0 Mbits/s Speed
• 2.5 Mbits/s Speed
Several different basic fieldbus topologies are possible.
These different topologies are defined and explained in
Section 5.0.
• Current Mode Connection
(AC Power Inductive Connection)
• Fiber Optic Medium
In Section 6.0, the concepts introduced in Section 4.0
are explained in more detail. This section is
recommended as a reference.
• Radio Medium
• Information related to non-fieldbus specific issues
Section 7.0 presents some application examples of how
fieldbus might be installed in a plant.
• General information on industry standard practices.
These issues will be addressed in additional Fieldbus
Foundation documents in the future.
Section 8.0 includes answers to some commonly asked
questions about fieldbus and Section 9.0 gives a
definition of most technical terms used in this document.
Section 10.0 provides some troubleshooting tips for
fieldbus networks.
The information in this manual is based on Physical
Layer Standards developed by the International Society
for Measurement and Control (ISA) and the International
Electrotechnical Commission (IEC). These standards are
identified in Section 11.0.
1
© 1996 Fieldbus Foundation, Austin, Texas. All rights reserved.
Installation
network. It can carry many process variables, as well
as other information. If you’ve got a cable with 9 wire
pairs, you can make nine fieldbus networks — if you
need that many. This definition of a network is
purposely narrow and includes only 31.25 kbit/s
devices and signaling. Devices operating at 31.25
kbit/s are sometimes referred to as H1 devices.
4.0 GETTING STARTED
Fieldbus is not difficult to understand and install. In this
section, we’ll explain what to do and give you a few easy
rules that should cover most installations.
The first thing to learn is
that some of the rules can be bent.
Figures 4.1-1 and 4.1-2 show how to make a fieldbus
network from an analog wire pair. In Figure 4.1-1 a
current loop connects an analog 4-20 mA field device to
a control system. Figure 4.1-2 shows the same wire pair
turned into a fieldbus network. The only changes are:
If you’ve read the IEC fieldbus standard, you know it’s
full of rules about things like cable lengths, cable types,
terminators, and so on. The first thing to learn is that
some of the rules can be bent. If you’re three minutes
late at the dentist, you can probably still get in. If you’ve
got 1950 m (6396 ft.) of fieldbus cable instead of 1900 m
(6232 ft.), your fieldbus should work. There are only a
few rules that are absolute. We’ll tell you which ones.
1. The control system’s 4-20 mA interface is replaced
with one that “talks” FOUNDATION fieldbus. We called
it a FOUNDATION Fieldbus Interface (FFI). This FFI
could, in fact, be in a personal computer or a PLC.
The justification for this philosophy is that fieldbus, like
most digital communication schemes, is subject to
performance variation based on a large number of
factors. Not all of these factors are likely to be at “worst
case” simultaneously.
2. The analog field device is replaced with one that
talks FOUNDATION fieldbus.
3. A terminator is added at the FFI end of the wire pair.
Another terminator is added at the field device end
of the wire pair. The FFI may have a built-in
terminator so that you don’t have to add one.
Check the manufacturer’s specifications to be sure.
4.1 Building the Network
In analog installations, two wires usually carry a signal
voltage or current to or from the field area. The
voltage or current on the wire pair represents one
process variable. In fieldbus the wire pair is called a
DCS
+
-
Notice that neither wire is grounded.
This is one of the absolute rules of fieldbus.
Wire Pair
+
Current
-
To A/D
Converter
Figure 4.1-1
4-20 mA Current Loop Connection
2
© 1996 Fieldbus Foundation, Austin, Texas. All rights reserved.
Analog
4-20 mA
Field
Device
Installation
In Figure 4.1-3 more field devices have been added to
the network of Figure 4.1-2. These new devices are
simply connected in parallel with the first field device.
The new devices are shown connected in a “star”
fashion. However, they could also be chained from the
first device as in Figure 4.1-4. All of the field devices
and the FFI in Figures 4.1-3 and 4.1-4 are said to be
“on the same network.” A detailed figure showing the
wiring of the junction of Figure 4.1-3 is given later.
You can see that new devices are always added in
parallel to existing ones. Notice that the number of
terminators in Figures 4.1-3 and 4.1-4 stays at two,
regardless of what else we add to the network.
FFI
Wire Pair
FOUNDATION
Fieldbus
Interface
Signals
T
Fieldbus
Digital
Field Device
T
Terminator
Terminator
Figure 4.1-2
Simple Fieldbus Network
FD
FD
FFI
Wire Pair
FOUNDATION
Fieldbus
Interface
FD
T
Signals
Terminator
T
Terminator
FD
FD = Field Device
Figure 4.1-3
Fieldbus Network With Additional Devices Added
3
© 1996 Fieldbus Foundation, Austin, Texas. All rights reserved.
Installation
terminator was placed at the junction of the group of
field devices. We’ve assumed that all of the field
devices were about the same distance (in cable length)
from the junction. Had one of them been a lot longer
than the others, then we would move the terminator
out to that device.
Speaking of terminators,
how did we know where to put them?
To answer this we need to define a trunk. A trunk is the
longest cable path between any two devices on the
network. Once we’ve identified this stretch of cable, all
other connections to it are called spurs. (More about
spurs later.) The terminators should be placed at the
ends of the trunk. We’ve assumed in Figures 4.1-3 and
4.1-4 that the FFI is further from the group of field
devices (in cable length) than they are from each other.
In Figure 4.1-4 we kept moving the terminator out to
the farthest field device each time we added a device.
However, if we are adding to an existing network and
all of the field devices to be added are located on one
short stretch of cable (100 m (328 ft.) or less), then the
terminator could have been left in its original position
at the first device.
This rule on location of terminators is one that can be
bent. In Figure 4.1-3, for example, we haven’t
bothered to find the longest cable path. Instead, the
Terminator
T
FFI
FD
FOUNDATION
Fieldbus
Interface
FD
T
FD
FD
Terminator
FD = Field Device
Figure 4.1-4
Fieldbus Network With Chained Devices
4
© 1996 Fieldbus Foundation, Austin, Texas. All rights reserved.
Installation
4.2 Adding to the Network
The total length is found as:
Trunk
We can add to the network by tapping into the trunk at
any point or by extending it. Suppose we want to add
a field device near the middle of the trunk of Figure
4.1-3, and a bridge to a high-speed network at the FFI
end. The result might be that of Figure 4.2-1 Notice
that the terminator at the FFI end has been moved to
the new end of the trunk. We can have a total of 32
devices on each segment of a network (even more
with repeaters), with some restrictions. These
restrictions will be considered next.
Length
600 m (1968 ft.)
Spur 1 Length
50 m (164 ft.)
Spur 2 Length
10 m (32.8 ft.)
Spur 3 Length
10 m (32.8 ft.)
Spur 4 Length
30 m (98.4 ft.)
Total
700 m (2296 ft.)
Length
This total is limited according to the type of cable
used. See Figure 6.1-1 for a summary of length
versus type of cable. For a total cable length of 700 m
(2296 ft.), we should be using either of types A or B
cable. Another restriction is spur length. This is
considered next.
One restriction is the total wire pair length in a given
segment. This is the sum of the trunk length and spur
lengths. In Figure 4.2-2, we’ve assigned some length
numbers to the network of Figure 4.2-1.
FD
FFI
FD
FD
FOUNDATION
Fieldbus
Interface
FD
T
FD
FD = Field Device
Bridge
T
Figure 4.2-1
Addition of a Device and a Bridge to Fieldbus Network
5
© 1996 Fieldbus Foundation, Austin, Texas. All rights reserved.
Installation
4.3 Spurs
reasonable to assume that, for 25 devices, we could
have something less than 30 m (98.4 ft.) but more than
1 m (3.28 ft.).
Shorter is Better!
A more common situation is to have all but one of the
devices satisfying the table. For example, suppose
we’ve got 14 devices, each with its own spur of
exactly 90 m; and a 15th device with a spur of 10 m
(32.8 ft.). Are we in trouble? Not likely. Our density of
spurs x length is 1270 m (4165 ft.). At 14 devices the
table allows us a density of 1260 m (4132 ft.) i.e. 14
devices x 90 m (295 ft.) per spur. We’ve exceeded the
table density by about 8%, which should be OK.
If you have a choice about the length of a spur, shorter
is better. The total spur length is limited according to
the number of spurs and number of devices per spur.
This is summarized in Table 6.11-1. A spur can be up
to 120 m (393.6 ft.) in length if there are few of them.
If there are 32 spurs, they should be 1 m (3.28 ft.) or
less. The spur table is not absolute. It merely tries to
establish some density of devices x length.
(Note: This density isn’t constant across all lengths
and numbers of spurs.)
Or suppose we’ve got 25 spurs of 1 m (3.28 ft.) each
and one spur of 120 m (393 ft.), connected in the star
configuration of Figure 4.1-3. In this case it’s
reasonable to assume that the one long spur is part of
the trunk and that we actually have 25 one-meter (3.28
feet) spurs and one “spur” of zero length. In this case
we may want to move the terminator to the end of the
120 m ( 393 ft.) cable section.
To some extent we can interpolate. Suppose, for
example, we have one device per spur and 25 spurs.
Going strictly by the table, we’re stuck with a length of
one meter.
If we could get rid of just one device, the table tells
us we could have 30 m (98.4 ft.). In this case it’s
FD
FFI
FD
FD
10 m
50 m
200 m
FOUNDATION
Fieldbus
Interface
10 m
300 m
FD
T
30 m
100 m
FD
FD = Field Device
Bridge
T
Figure 4.2-2
Segment Lengths for New Fieldbus Network
6
© 1996 Fieldbus Foundation, Austin, Texas. All rights reserved.
Installation
4.4 Repeaters
LX / MAXX + LY / MAXY < 1
where:
What if you need a lot more than 1900 m (6232 ft.) of
cable? You can do it by using a repeater. The repeater
takes the place of one of the field devices. But it
means you get to start fresh. You can add another
1900 m (6232 ft.) of cable, as illustrated in Figure 4.4-1.
Notice that a new trunk has been created so that we
have to add more terminators. The first trunk has 4
devices, one of them being the repeater. The 2nd trunk
has 2 devices, one of them being the repeater. You can
use up to 4 repeaters in series between any two
devices to get a total length of 9500 m (6232 ft.).
LX = length of cable X
LY = length of cable Y
MAXX = maximum length for X alone
MAXY = maximum length for Y alone
As an example, suppose we want to mix 1200 m
(3936 ft.) of type A cable with 170 m (557 ft.) of type D.
Then we have:
LX = 1200 m (3936 ft.)
LY = 170 m (557 ft.)
MAXX = 1900 m (6232 ft.)
In addition to increasing the length of a network,
repeaters can be used to increase the number of
devices in a network beyond the limit of 32 on one
segment. Using repeaters, the maximum number of
devices in a network could be increased to 240.
MAXY = 200 m (656 ft.)
1200 / 1900 + 170 / 200 = 1.48
Since the result is > 1 we shouldn’t do it. The formula
shows that 170 m (558 ft.) of type D and 285 m (935 ft.)
of type A would be OK, since this gives a value of
exactly 1.
4.5 Mixing Cables
Occasionally you might need to mix cable types. The
maximum lengths for the two types are determined
according to a formula:
FFI
1st Trunk
FD
FD
T
T
FOUNDATION
Fieldbus
Interface
Repeater
T
FD
FD
T
FD = Field Device
Figure 4.4-1
Adding a Repeater to the Network
7
© 1996 Fieldbus Foundation, Austin, Texas. All rights reserved.
2nd Trunk
Installation
This idea is easily extended to four cable types.
The formula for four types is
output fieldbus power supply (if anyone makes them) but it illustrates the need to consider Ohms Law effects.
Another solution, of course, is to use a repeater as
discussed in Section 4.4.
LV/MAXV + LW/MAXW + LX/MAXX + LY/MAXY < 1
Note that it doesn’t matter where in the network the
two (or more) types of cable are.
4.6 Shielding (Screening)
So far, we’ve only talked about wire pairs. A fieldbus
network can be built using only unshielded wire pairs
(type C cable). If these are placed in conduit or laid
against a metal surface, there may be sufficient
shielding that nothing further need be done.
For communications purposes, the combined lengths
of cable should work OK but this is probably not the
only limitation. In systems of bus-powered devices,
for instance, it may not be possible to supply devices
at the opposite end of the bus from the power supply
with the operating voltage and current they require simply due to the effects of Ohms Law and the cable
resistance.
However, for best performance, fieldbus cables should
be shielded. Common multi-conductor (multi-core)
“instrument” cable can be used. This has one or more
twisted pairs, an overall, metalized shield, and a shield
wire. An example is the single-pair cable illustrated in
Figure 4.6-1. Lugs have been added to the wires in
anticipation of connecting them to a terminal block.
You can also use cable that has individually shielded
pairs. For new installations, ask cable vendors for
“fieldbus cable.”
For example, using a bus made from 190 m (623 ft.) of
Type “A” and 360 m (1181 ft.) of Type “C” cable (which
is allowed from the formula) and a fieldbus power
supply having a 24V output, then the overall (loop)
resistance of the bus will be 2*((0.19*24)+(0.36*132)) =
104 Ohms. So, to provide a voltage of at least 9V at the
far end, the bus can supply a maximum current of
1000*(24-9)/104 = 144 mA to bus-powered devices
connected at this far end. Assuming each device
consumed about 14 mA, then there could be 10
devices on this segment. There are ways of improving
this - for example, with a 32V
When using shielded cable, connect each
spur’s shield to the trunk shield and connect
the overall shield to ground at one point.
Figure 4.6-1
Single-Pair Fieldbus Cable
8
© 1996 Fieldbus Foundation, Austin, Texas. All rights reserved.
Installation
For most networks the ground point can be anywhere.
Figure 4.6-2 is a re-construction of Figure 4.1-3 using
shielded cable. The grounding point has been chosen
as the junction of the field instruments. Figure 4.6-3 is
a detailed look at the wiring of the junction. For
intrinsically safe (I.S.) installations, a specific location
may be required for the ground. (See Section 4.9 on
intrinsic safety.)
Note: In some instances, better high-frequency EMI
shielding requires that the shield be connected to
ground at multiple points. (See Mardiguian, M., and
White, D.R.J., EMI Control Methodology and
Procedures.) Fieldbus provides for this by allowing an
rf ground at multiple points, consisting of a small
capacitor from shield to ground.
FD
FD
FFI
T
FOUNDATION
Fieldbus
Interface
FD
T
FD = Field Device
FD
Figure 4.6-2
Use of Shielded Cable
Figure 4.6-3
Field Instrument Wiring Junction
9
© 1996 Fieldbus Foundation, Austin, Texas. All rights reserved.
Installation
4.7 Polarity
If you build a fieldbus network to accept all possible
devices, you must take the signal polarity into
account. All of the (+) terminals must be connected to
each other. Similarly, all of the (-) terminals must be
connected to each other. Color-coded wire makes
this relatively easy. Polarized devices should always be
marked or have keyed connectors. Non-polarized
devices might not be marked.
The Manchester signal used by fieldbus is an
alternating voltage that changes polarity once or twice
per bit. In unpowered networks only this alternating
voltage exists. In powered networks the alternating
voltage is superimposed onto the DC voltage being
used to power the devices. In either case, the fieldbus
receive circuits look at only the alternating voltage.
Positive voltage swings have one meaning, negative
swings have the opposite meaning. Therefore, the
fieldbus signal is polarized. Field devices must be
connected so that they all see the signal in correct
polarity. If a field device is connected “backwards” it
will see an inverted version of the alternating voltage
and won’t be able to communicate.
It may be possible to select devices with the intention
of never having to consider polarity. But it may be
safer to build polarity into the network at the outset
and accept as a bonus any devices that can be
blindly connected. This approach also lets you
expand the network with polarized devices if you
need them later on.
Bus powered field devices may also be polarized
with respect to device power. These devices are
designed so that signal polarity and power polarity
are the same. Connecting the +DC to the (+) terminal
automatically insures correct signal polarity. Nonpolar bus powered field devices accept both signal
and power of either polarity.
Now that we know that fieldbus is polarized, it may be
a surprise to learn that there are non-polarized field
devices. You can connect them in either polarity
across the network. The non-polarized devices are
invariably network-powered, which provides a clue as
to how they can work with either polarity: They
effectively sense the network DC voltage. In so doing
they “know” which terminal is which. Another step up
in sophistication is a device that implements automatic
polarity detection and correction. It will correctly
receive a message of either polarity!
10
© 1996 Fieldbus Foundation, Austin, Texas. All rights reserved.
Installation
4.8 DC Power for Two-Wire Field Devices
If you have 2-wire field devices in your network, you
have to make sure they have enough voltage to
operate. Each device should have at least 9 volts.
You need to know:
Some field devices draw operating power from the
fieldbus network in much the same way as 2-wire
analog field devices. The power supply is connected
to the network in the same way as a field device (or
other communicating device). All of the same rules
apply, except that you don’t have to count the power
supply as one of the 32 field devices. If we add a
power supply to the network of Figure 4.1-3, it might
look something like that of Figure 4.8-1. Another spur
near the FFI has been created to add the power
supply. Of course we could have put it toward the
field end of the trunk.
1. The current consumption of each device.
2. Its location on the network.
3. The location of the power supply on the network.
4. The resistance of each cable section.
5. The power supply voltage.
The voltage at each field device is determined through
straightforward DC circuit analysis. This will be done
for the example network in Figure 4.2-2.
We can’t use just any off-the-shelf
power supply, because it would short-circuit
the (digital) fieldbus signals.
It is assumed that a 12 volt power supply is built into
the FFI and that type B cable is used throughout. One
of the field devices on a 10 m (32.8 ft.) spur is also
assumed to be separately powered (a 4-wire device).
The other of the field devices on a 10 m (32.8 ft.) spur
draws 20 mA. The remaining three field devices draw
10 mA each. The bridge is separately powered and
draws no network current.
The power supply is specially designed for fieldbus.
Some fieldbus equipment will have a built-in power
supply so that you don’t need to add one. You should
consult manufacturers’ specifications.
FFI
Fieldbus
Power
Supply
+
FD
-
FD
Wire Pair
FOUNDATION
Fieldbus
Interface
FD
T
Signals
Terminator
T
Terminator
FD
FD = Field Device
Figure 4.8-1
Adding a Power Supply to the Network
11
© 1996 Fieldbus Foundation, Austin, Texas. All rights reserved.
Installation
Neglecting temperature effects, the resistance per
meter is 0.1 Ohm. The following table shows the
resistance of each section, the current through the
section, and the resulting voltage drop across the
section.
Section Length
(meters (feet))
Figure 4.8-2 shows these voltages. Since all field
devices have at least 9 volts available, this should be
satisfactory. In networks with more field devices or
smaller gauge cable this may not always be the case.
Clearly, this is a tedious and troublesome process.
And adding one more network-powered field device
forces you to do it again. Fortunately, fieldbus
equipment vendors provide computer software that
makes it easy. You just input your proposed network
and all of the DC voltages are immediately presented.
If you change the network, the voltages are
immediately re-calculated.
Resistance Current Voltage Drop
(Ohms)
(Amps)
(Volts)
200 (656)
20
0.05
1.0
50 (164)
5
0.01
0.05
300 (984)
30
0.04
1.2
10 (32.8)
1
0.02
0.02
30 (98.4)
3
0.01
0.03
In some cases the network may be so heavily loaded
that you can’t add a full 12 network-powered field
devices. Or you may have to relocate the power
supply.
The voltages at the network-powered devices are
as follows:
Spur Length
(meters (feet))
Voltage
(Volts)
50 (164)
10.95
Trunk End
9.80
10 (32.8)
9.78
30 (98.4)
9.77
When doing these calculations, be sure to account for
increased cable resistance at high temperatures.
If your 2-wire field devices are polarized, make sure
you connect them to the network in correct polarity.
You can easily test the completed network for sufficient
voltage by connecting a DVM across the 2-wire field
device terminals. The DVM can be connected to an
operating network, provided that it is a batteryoperated type with no connections to ground.
9.78 Volt
FD
FFI
FD
10.95 Volt
FD
12 Volt
FOUNDATION
Fieldbus
Interface
FD
T
9.77 Volt
9.77 Volt
Bridge
FD
T
Figure 4.8-2
Voltages at Network Powered Devices
12
© 1996 Fieldbus Foundation, Austin, Texas. All rights reserved.
Installation
4.9 Intrinsic Safety
If you’re installing intrinsically safe field devices, there
are more things to consider. The number of field
devices may be limited due to power limitations. A
special fieldbus barrier and special terminators may
be required. The amount of cable may be limited due
to its capacitance or inductance per unit length. An
excellent source for additional information on the
subject of Intrinsic Safety is the “Application Guide for
Intrinsic Safety.” This document is available from the
Fieldbus Foundation. Refer to Product Code AG-163
when ordering.
networks, etc. If it doesn’t say “Fieldbus” somewhere
on it, then it’s suspect. (A battery-operated voltmeter
can be temporarily connected to check voltages.) If a
device is capable of several optional wiring
arrangements, then:
Read the manual before connecting it!
4.12 Connecting to Higher Speed Fieldbus
Networks
So far we’ve been discussing only 31.25 kbit/s fieldbus
networks. These networks may be part of a larger
network operating at speeds of 1.0 or 2.5 Mbit/s. The
31.25 kbit/s network must never be connected directly
to a higher speed network. A special device called a
bridge must be placed between them. The bridge will
have a 31.25 kbit/s port and a high speed (i.e. 1.0 Mbit/s
or 2.5 Mbit/s) port to facilitate these connections. The
bridge must be counted as one of the 32 field devices
on each network.
4.10 Live Wire
Fieldbus devices are designed for connection to a
live network. This is done so that the network need
not be shut down to service a device. Devices are
designed so that they aren’t harmed by this. However,
the communication can suffer. To ensure minimal
disruption to communication, take care to insulate
each network wire immediately after it is disconnected
from the device; and take every precaution against
either wire coming into contact with any other
conductor.
4.13 What If It’s Not Wired Correctly?
The nature of digital communication systems
(including fieldbus) is that they slow down if you don’t
have things quite right. That’s because a given device
has to keep sending the same message until it gets
through. If a master device or bus analyzer tells you
that there are numerous retries, this is a clue that
something’s wrong. Note: You don’t have to worry
that something will be misprogrammed. The built-in
error detection prevents this.
Remember: If either wire is grounded or if the
two wires touch each other, nobody can talk!
4.11 What Not to Connect
Although it’s probably obvious, you can’t connect nonfieldbus devices to the network. Examples would be
light bulbs, analog 4-20 mA field devices, other
13
© 1996 Fieldbus Foundation, Austin, Texas. All rights reserved.
Topologies
two), or it could be a field device (transmitter) connected
to a host system (doing control or monitoring). Simple
point-to-point (host and one device per bus segment),
would probably not be often used as it has only one
measurement OR control device per segment (as in 420 mA). As a result it doesn’t take advantage of the
multi-device-per-bus-segment capability. This topology
is illustrated in Figure 5.1-1.
5.0 FIELDBUS TOPOLOGIES
There are several possible topologies for fieldbus
networks. This section will illustrate some of these
possible topologies and discuss some of the
characteristics of each. In the interest of clarity and
simplicity, power supplies and terminators are omitted
from the diagrams in this section.
Figure 5.0-1 shows the four topologies that will be
discussed in detail in the following subsections.
5.2 Bus with Spurs Topology
With this topology, the fieldbus devices are connected
to the bus segment through a length of cable called a
spur. A spur can vary in length from 1 m (3.28 ft.) to
120 m (394 ft.). A spur that is less than 1 m (3.28 ft.) in
length is considered a splice. This topology is
illustrated in Figure 5.2-1.
5.1 Point-to-Point Topology
This topology consists of a segment having only two
devices. The segment could be entirely in the field (a
slave and host device operating independently - e.g. a
transmitter and valve, with no connection beyond the
Control Highway
(Terminators and Power
Supplies Not Shown)
Fieldbus I/O
Junction
Box
Point to Point
Bus with spurs
(or drops)
Daisy Chain
Tree
Figure 5.0-1
Possible Fieldbus Topologies
J-Box
J-Box
Single or multi-pair cable
(Terminators and Power
Supplies not shown)
Junction
Boxes
Figure 5.1-1
Simple Point-to-Point Topology
Figure 5.2-1
Bus with Spurs Topology
14
© 1996 Fieldbus Foundation, Austin, Texas. All rights reserved.
Topologies
5.3 Daisy-Chain
With this topology, the fieldbus cable is routed from
device to device on this segment, and is interconnected
at the terminals of each fieldbus device. Installations
using this topology should use connectors or wiring
practices such that disconnection of a single device is
possible without disrupting the continuity of the whole
segment.
cable. It is practical if devices on the same segment
are well separated, but in the general area of the same
junction box. When using this topology, the maximum
spur length must be taken into consideration.
Maximum spur lengths are discussed in Section 6.0.
This topology is illustrated in Figure 5.4-1
5.5 Combinations of the Above
This topology is illustrated in Figure 5.3-1.
Combinations of the above must follow all the rules for
maximum fieldbus segment length, and include the
length of spurs in the total length calculation. Although
probably unlikely, the following figure indicates a
combined topology that could occur.
5.4 Tree Topology
With this topology, devices on a single fieldbus
segment are connected via individual twisted wire
pairs to a common junction box, terminal, marshaling
panel, or I/O card (sometimes called “Chicken Foot”).
This topology can be used at the end of a home run
An example of a configuration combining these
topologies is given in Figure 5.1-1.
Fieldbus
Interface
Figure 5.3-1
Daisy Chain Topology
Junction
Box
Junction
Box
Control Highway
Fieldbus I/O
Terminate
Here
Terminate
Here
Junction
Box
Fieldbus
Host
Bus with spurs
Figure 5.4-1
Tree Topology
Point to Point
Figure 5.5-1
Mixed Topology
15
© 1996 Fieldbus Foundation, Austin, Texas. All rights reserved.
Tree
Components
6.1 Cable
6.0 FIELDBUS COMPONENTS
AND CHARACTERISTICS
Various types are useable for fieldbus. The table in
Figure 6.1-1 contains the types of cable identified by
the IEC/ISA Physical Layer Standard.
This section presents a detailed look at the following
fieldbus segment components:
The preferred fieldbus cable is specified in the IEC/ISA
Physical Layer Standard, Clause 22.7.2 for
conformance testing, and it is referred to as type “A”
fieldbus cable. (This cable will probably be used in
new installations.)
Cable
Terminal Blocks
Connectors (optional)
Couplers (optional)
Other types of cables can be used for fieldbus wiring.
The alternate preferred fieldbus cable is a multiple,
twisted pair cable with an overall shield, hereafter
referred to as Type “B” fieldbus cable. (This cable will
probably be used in both new and retrofit installations
where multiple fieldbuses are run in the same area of
the user’s plant.)
Terminators
Power Supplies
I. S. Barriers
Measurement & Control Devices
A less preferred fieldbus cable is a single or multiple,
twisted pair cable without any shield, hereafter referred
to as Type “C” fieldbus cable. The least preferred
fieldbus cable is a multiple conductor cable without
twisted pairs, but with overall shield, hereafter referred
to as Type “D” fieldbus cable. Types “C” and “D”
cables will mainly be used in retrofit applications. They
will have some limitations in fieldbus distance as
compared to the Type “A” and “B.” This may preclude
the use of Type “C” and “D” cables in certain
applications.
Host Devices
Bridges & Repeaters
Additional information on determining allowable cable
lengths is given in Sections 4.2 through 4.5.
Type
Cable Description
Size
Max Length
Type A
Shielded, twisted pair
#18 AWG
(.8 mm2)
1900 m
(6232 ft.)
Type B
Multi-twisted-pair with shield
#22 AWG
(.32 mm2)
1200 m
(3936 ft.)
Type C
Multi-twisted pair, without shield
#26 AWG
(.13 mm2)
400 m
(1312 ft.)
Type D
Multi-core, without twisted pairs
and having an overall shield
#16 AWG
(1.25 mm2)
200 m
(656 ft.)
Figure 6.1-1
Fieldbus Cable Types and Maximum Lengths
16
© 1996 Fieldbus Foundation, Austin, Texas. All rights reserved.
Components
6.2 Terminal Blocks
6.4 Couplers (Optional)
Terminal blocks can be the same terminal blocks as
used for 4-20 mA. The terminal blocks typically
provide multiple bus connections, such that a device
can be wired to any set of bus terminals.
Figure 6.2-1 indicates one method of connecting and
terminating a fieldbus segment to several field devices
at a junction box using the type of terminal blocks that
have been used in the past. Today, there are terminal
block systems especially designed for fieldbus that
make wiring considerably easier.
Fieldbus couplers are specified in the IEC/ISA
Physical Layer Standards, and provide one or several
points of connection to a fieldbus segment. In
general, for the fieldbus technology covered by this
document, the end user will not need to be
concerned with couplers. Please refer to the
references (para 7.1) for further details.
6.5 Terminators
A terminator is an impedance matching module used
at or near each end of a transmission line. There need
to be two (and ONLY TWO) terminators per bus
segment. The terminators prevent distortion and signal
loss, and are typically purchased and installed as a preassembled, sealed module. The user/installer need not
be concerned about or assemble individual electrical
resistors and capacitors. Various mechanical and
electrical configurations will incorporate terminators
into terminal blocks, I.S. Barriers, power supplies,
fieldbus interface cards, etc.
6.3 Connectors (Optional)
Standard fieldbus connectors are specified in Annex B
of the ISA Physical Layer Standard and Annex A of the
IEC Physical Layer Standard. They will not be
discussed in detail here.
They are useful for installations where devices may be
periodically disconnected and/or moved, and could be
used for convenient connection of a temporary device
(e.g. “handheld” configuration device) at a frequently
used location.
Figure 6.2-1
Terminal Block in Field Mounted Junction Box
17
© 1996 Fieldbus Foundation, Austin, Texas. All rights reserved.
Components
6.6 Power Supplies
6.8 Grounding, Shields, Polarity
Power supplies are designated as the following types
per the referenced Fieldbus Foundation Physical Layer
Profile Specification:
type 131
Non-I.S. Power supply intended for
feeding an I.S barrier. Output voltage
depends on barrier rating.
type 132
Non-I.S. Power supply not intended
for feeding an I.S. barrier. Output
voltage is 32 VDC Max.
type 133
I.S. Power supply. Complies with the
recommended I.S. parameters.
Grounding: It is the assumption of this application
note that grounding rules used in conjunction with the
installation and maintenance of the Fieldbus Physical
Layer will follow current standard practices based on
company/plant standards and applicable international
standards (see references, par 7.1).
Fieldbus devices shall not connect either conductor of
the twisted pair to ground at any point in the network.
The Fieldbus signals are applied and preserved
differentially throughout the network, and grounding
either conductor would be expected to cause all
devices on that bus to lose communication for the
period the conductor is grounded.
For fieldbus use, a power supply impedance matching
network is required. This is a resistive/inductive
network that is either external or built into the fieldbus
power supply.
Shields: The Shield of the fieldbus cable is, by
standard practice, typically grounded at only one point
along the length of the cable, and shall not be used as
a power conductor.
Power supplies may be configured to provide dualredundancy as long as they meet the IEC/ISA Physical
Layer Standards requirements. It is not acceptable,
for example, to merely parallel two power supplies.
It is standard practice in some plants to ground the
same shield at more than one point over the length
of the cable run. This practice may be acceptable in
4-20 mA DC control loops, but can cause interference
in a fieldbus system. If a multi-point grounding
scheme is used as standard practice, the
requirements for and location of grounds for cable
shields should be re-visited.
6.7 Intrinsic Safety Issues
Any fieldbus segment used in a potentially flammable
atmosphere must meet the applicable requirements of
the relevant regulatory agencies of the local area. This
will normally involve using an I.S. barrier or galvanic
isolator.
There may be specific requirements for I.S.
installations. Please refer to the documents
referenced in Section 6.7 for more detail.
The I.S. barriers are designed and certified to meet all
of the Physical Layer requirements. This document
will not address I.S.-specific issues in detail, as the
appropriate standards and documents are referenced.
Please see par 7.1 for reference to detailed documents
on I.S. issues. Additional information on the subject
of Intrinsic Safety is also given in the Fieldbus
Foundations’ publication AG-163 entitled “Application
Guide on Intrinsic Safety”.
Polarity: All systems addressed by this application
note may be polarity sensitive. Based on the
referenced physical layer standards, there is not a
requirement for devices to be polarity insensitive,
although some devices may be designed as polarity
insensitive. The medium (twisted pair) is required to
indicate polarity, and polarity shall be maintained at all
connection points (per the physical layer standard).
18
© 1996 Fieldbus Foundation, Austin, Texas. All rights reserved.
Components
6.9 FOUNDATION Device Types
Repeaters are active bus powered, or non-bus
powered devices, used to extend a fieldbus network.
The eight types of 31.25 kbit/s fieldbus devices (as
designated by the referenced Fieldbus Foundation
Physical Layer Profile Specification) are categorized in
Figure 6.9-1 below.
A maximum of four (4) repeaters and/or active
couplers can be used between any two devices on a
fieldbus network (IEC/ISA Physical Layer Standard,
Section 22.2.2, “Rule 3”). Using four repeaters, the
maximum distance between any two devices on that
network is 9500 metres (31,168 ft. or 5.9 miles) as
indicated in Figure 6.10-1.
Note: Bus powered devices require a compatible
power supply. See Section 6.6 of this document.
FOUNDATION devices will be labeled with their Fieldbus
Foundation type.
A Bridge is an active bus powered, or non-bus
powered device, used to connect fieldbus segments
of different speeds (and/or physical layers - e.g. wire,
optical fiber,..) together to form a larger network.*
From a communications standpoint, any of the eight
device types can coexist on the same fieldbus segment.
However, in I.S. applications, device power requirements
and approvals must be taken into consideration. This, in
effect, eliminates four of these types.
A Gateway is an active bus powered, or non-bus
powered device, used to connect a fieldbus segment
or segments to other types of communications
protocols (e.g. Ethernet, RS232, ..).*
6.10 Special Communications Devices
(Repeaters, Bridges, and Gateways)
* It is beyond the scope of this application note to
discuss these devices in detail. Refer to other
Fieldbus Foundation publications for more information
on these topics.
Other devices such as repeaters, bridges, and
gateways may be connected to a bus segment. They
will have two wire connections (or may be separately
powered) and will appear just as any other device on
the segment.
I.S.
Non I.S.
Clause 11 Standard Signaling
Bus Powered
Separately
Powered
Type 111
Type 112
Type 113
Type 114
Clause 22 Low Power Signaling
Bus Powered
Separately
Powered
Type 121
Type 122
Type 123
Type 124
Figure 6.9-1
Fieldbus Device Types
Figure 6.10-1
Maximum Use of Fieldbus Repeaters
19
© 1996 Fieldbus Foundation, Austin, Texas. All rights reserved.
Components
6.11 Maximum Cable Segment Lengths and
Elements per Segment
cable types and the maximum bus segment length
values from the table given in Figure 6.1. A less
conservative approach is to use the techniques
discussed in Section 4.5.
The maximum lengths indicated in the specifications
are recommendations, containing a safety factor, to
provide assurance that the suggested maximum
lengths will not cause communications problems. The
recommended segment length depends on:
Allowable spur lengths for either bus or tree topology
are dependent on the number of communication
elements on the fieldbus.
Figure 6.11-1 relates the recommended number of
communication elements to spur length. Maximum
spur lengths are the same for Types A, B, C, and D
cables. The table assumes one communication
element per spur. When a spur with passive trunk
coupler has more than one communication element,
the length of that spur should be reduced by 30 m (98
ft.) per communication element. As the recommended
maximum total spur length is 120 m (394 ft.), the
maximum number of communication elements per
spur should be 4.
• Cable type/characteristics/wire gage
• Topology
• Number and type of devices
Cable types were discussed in Section 6.1 and the
different topologies were presented in Section 5.0.
Most likely, there will be a mixture of cable types in any
fieldbus segment. The conservative approach is to
use the shortest maximum length based on all the
Total Devices
1 Device per Spur
2 Devices per Spur
3 Devices per Spur
4 Devices per Spur
25-32
1 (3)
1 (3)
1 (3)
1 (3)
19-24
30 (98)
1 (3)
1 (3)
1 (3)
15-18
60 (197)
30 (98)
1 (3)
1 (3)
13-14
90 (295)
60 (197)
30 (98)
1 (3)
1-12
120 (394)
90 (295)
60 (197)
30 (98)
Figure 6.11-1
Recommended Maximum Spur Length, m(ft)
• Source: IEC-1158-2 and ISA S50.02-1992 Part 2, Annex C (informative).
NOTE: These lengths are “recommended” and are not required.
20
© 1996 Fieldbus Foundation, Austin, Texas. All rights reserved.
Examples
The plant designer chooses to use one network for
each of these control functions and to use a personal
computer as the fieldbus host and monitor.
7.0 EXAMPLE FIELDBUS APPLICATIONS
This section presents two actual examples of how a
fieldbus network might be designed and implemented.
The choice of the number of networks is dependent on:
7.1 Waste Treatment Plant Example
1. Fieldbus topology rules (the things we’ve been
discussing).
A diagram of a typical water treatment plant is shown
in Figure 7.1-1. This diagram has been previously
published by the Fieldbus Foundation. In this
application, waste water of variable pH is to be mixed
with acid or base to neutralize it before discharging it
into a river. The control building is located near a river,
which is about 800 m (1/2 mile) from the nearest road.
The longest distance between any two plant facilities
at the river is 182 m (600 ft.). The acid and base
materials are transferred from trucks that are on or
near the road. A P&ID diagram for the plant is given in
Figure 7.1-2.
2. Control Logistics (see below).
3. Personal preference (what works best for you).
Control logistics is generally outside the scope of this
discussion. Briefly, however, it deals with making it
easy for a feedback signal to get to where it’s
supposed to go. For example, flow transmitter FT101
and pH transmitter AT102 both provide feedback to
valves IP103A and IP103B (see Figure 7.0-2). So it
helps if all four of these devices are on the same
network. The control is implemented by telling each
IP to be a “subscriber” and each transmitter to be a
“publisher.” Control logistics also deals with making
sure that feedback signals get there on time, which
may limit the number of devices or traffic on a
network.
There are basically 2 control requirements:
1. Controlling filling and emptying of the acid and base
storage tanks.
2. Controlling the movement of the waste water and
adjusting its pH.
Figure 7.1-1
Typical Waste Treatment Plant
21
© 1996 Fieldbus Foundation, Austin, Texas. All rights reserved.
Examples
The plant designer assigns devices to the two
networks as follows:
Fieldbus Device
Network 1
Network 2
VALVES
CV-108
CV-109
CV103A
CV103B
15
16
18
19
13
14
PUMPS
MOTORS
XMITTERS
NOTATION:
CV = control valve
LT = level transmitter
AT = analytical (pH) transmitter
TT = temperature transmitter
The resulting networks are shown in figure 7.1-3.
Network 1 is “Segment #1.” Network 2 is “Segment
#2.” The network cable is identified in each case by a
squiggly line that connects to each of the network
devices. Network 1 starts out at the Control Room PC
and snakes its way over to valve CV-108. From there
it goes to each of the Network 1 devices and finally
ends up at pump 18. Similarly Network 2 starts out at
the PC and goes to pumps 13 and 14. It ends up at
temperature transmitter TT107.
17
LT-108
LT-109
FT101
AT102
AT103A
AT103B
LT104
AT105
AT106A
AT106B
TT107
Figure 7.1-2
P & ID Diagram for Waste Treatment Plant
22
© 1996 Fieldbus Foundation, Austin, Texas. All rights reserved.
Examples
Now let’s see how things are starting to stack up in
terms of the topology rules. First, look at the number of
devices: 9 for Network 1 and 15 for Network 2
(Remember to count the PC in both networks). Both are
within the limit of 32. What about Network 2 and the
limit of 12 network-powered devices? We haven’t said
yet which devices, if any, are network-powered. Let’s
assume the two control valves of Network 2 are
separately powered. That leaves us with 13 devices —
just over the limit. Rather than add another network, the
plant designer has decided to exceed the limit by 1
device. Power calculations show that this should be OK.
Since this is a new installation, the plant designer has
chosen to use type A cable inside conduit throughout;
and to use a bus arrangement. That is, the trunk cable
snakes around to and is connected directly to each of
the devices. There either are no spurs or their length is
insignificant.
Last, but not least, are the terminators. Four are required.
On Network 1 one of the terminators is located at the PC.
The other is located at pump 18. For Network 2, one
terminator is located at the PC. The other is located at
transmitter TT107. The plant designer has chosen PC
cards (one for each network) that have built-in terminators,
so that terminators don’t need to be added at the PC end.
At the field end of Network 1 a terminator is added across
the network inside the wiring compartment of pump 18.
How about total length? Let’s neglect spurs for the
moment. Network 1 has to reach to the road because
pumps 18 and 19 are located at the road. So its length
is 800 m (2624 ft.). Network 2 snakes around to various
devices that are spaced, at most, 182 m (597 ft.) from
each other. We assume that there could be as much as
250 m (820 ft.) here. Network 1 could use cable types A
or B, while Network 2 could use types A, B, or C.
At the field end of Network 2 the wiring compartment of
TT107 is not large enough to hold the terminator.
Instead, the wiring of TT107 is completed in a junction
box and the terminator is placed in the junction box.
Figure 7.1-3
Fieldbus Network for Waste Treatment Plant Example
23
© 1996 Fieldbus Foundation, Austin, Texas. All rights reserved.
Examples
7.2 Paper Mill Retrofit Example
The second example is that of retrofitting a paper mill.
Figure 7.2-1 is a diagram of that mill.
area of Figure 16. It mixes together 6 ingredients: (1)
recycle fiber, (2) broke, (3) starch, (4) clay, (5) TiO2, and
(6) softwood. These must be mixed in a controlled
ratio, depending on the type of paper to be produced.
To keep the example manageable, we’ll consider only
the operation of the blend chest. This is the upper left
A P&ID diagram of the blend chest is shown in Figure
7.2-2.
Figure 7.2-1
Diagram of Typical Paper Mill
Figure 7.2-2
P & ID Diagram of Typical Paper Mill Example
24
© 1996 Fieldbus Foundation, Austin, Texas. All rights reserved.
Examples
There are 13 control elements consisting of one level
transmitter, 6 flow transmitters, and 6 valves. In the
existing plant, these are all connected through 4-20
mA wire pairs to a DCS which sets the blend chest
level and controls the mixing ratios. The wire pairs are
all brought to a junction box. A multi-pair cable
connects the junction box to the DCS. Figure 7.2-3
shows a wiring diagram, along with cable gauges and
estimates of the wire length.
4-20 mA interfaces would also be replaced. The 13
independent wire-pairs are to be replaced with one
fieldbus network, if possible, assuming the user is
comfortable with six valves on one bus. The existing
device-to-junction box cables would be left in place.
One of the 13 wire pairs in the home run cable would
be used as the connection from the junction box to the
DCS. The remaining 12 wire pairs in this cable
become spares. The proposed wiring diagram is
shown in Figure 7.2-4.
The basic plan is to remove the 13 field devices and
replace them with fieldbus equivalents. The DCS
Figure 7.2-3
Original Wiring Diagram for Paper Mill Example
25
© 1996 Fieldbus Foundation, Austin, Texas. All rights reserved.
Examples
The existing cable is already fieldbus type B, which
means that the total length can be 1200 m (3936 ft.).
In Figures 7.2-3 and 7.2-4 the cable shields have been
left out for clarity. The existing total of 890 m (2919 ft.)
is OK. Also, for 14 devices (Remember to count the
DCS as a device) the allowed spur length is 90 m (295
ft.), which means that each 30 m (98 ft.) cable from
junction to device can be considered a spur. The
home run becomes the trunk. To complete the wiring,
one terminator would be placed inside the junction
box. Another would be placed at the DCS.
doesn’t work for multiple fieldbus devices!
An unusual solution in this instance would be to
connect a repeater with a fieldbus power supply at
the junction box. Since each spur has less than an
ohm of resistance, the cable voltage drop becomes
insignificant and each field device has almost the full
supply voltage.
Another solution is to back off from the plan of making
a single network. Assuming a 24 volt supply, each
existing pair can supply power to about 3 devices (60
mA x 0.26 ohm/meter x 890 meter = 14 volts). Five
networks would be required for 13 devices. At the
control end, we can’t connect the 5 pairs together in
an attempt to get back to one network. This would
grossly exceed the 1200 m (3936 ft.) length limit. We
can, however, combine all 5 networks at the H2 level
by connecting each pair through a bridge to a single
H2 network. Or, one could connect each of 4 pairs by
a repeater to the 5th pair, to form a single H1 network.
With topology constraints apparently satisfied, the plant
manager proceeds with the plan and orders the field
devices. Unfortunately, each is a 2-wire device with a
current consumption of 20 mA. The total current is then
0.26 amp. Since #26 gauge cable has a resistance of
0.26 ohm/meter, the voltage drop in the home run cable
will be 33.8 volts. This is too high a voltage drop. The
#26 cable was fine for a single 4-20 mA device, but it
Figure 7.2-4
Proposed Fieldbus Wiring for Paper Mill
26
© 1996 Fieldbus Foundation, Austin, Texas. All rights reserved.
Questions & Answers
8.0 QUESTIONS AND ANSWERS
Q: How can I get more specific information on how
to start - relative to my particular plant, application,
and installed hardware?
A: Contact the Fieldbus Foundation (see references par 8.0) or your suppliers of choice. Training courses,
additional reference materials, and consulting services
are available to provide more detailed information on
specific products from the appropriate supplier.
Q: Can I use my installed wire?
A: Very likely. The IEC/ISA Physical Layer Specification
was purposely written such that existing plant wire
would work for fieldbus. The specification was
purposely written in a conservative fashion, so that
installed wire in reasonably good condition should offer
totally acceptable performance. In some cases
(especially for very long wire runs, and/or many devices
at the end of a segment), the user may choose to install
new type “A” fieldbus wire to minimize the effects of
resistance and capacitive unbalance which may come
into play using types “B” through “D” cable.
Q: Where do I get the specs?
A: The specifications (and additional documents) are
available from the Fieldbus Foundation. Standards
documents are available from the ISA and IEC.
Q: How can I determine if the installed wire will
work? Is there a test? Is there any test equipment
I can rent, buy,...?
A: A number of suppliers are developing products for
Q: Where can I buy the correct wire, terminals,
terminations, and other items required to install a
fieldbus?
A: The Fieldbus Catalog available from the Fieldbus
Foundation contains references to the most
comprehensive list of suppliers, devices, services, and
other fieldbus related items.
Q: Where can I go to get assistance in planning
and installing a Fieldbus Physical Layer for a “trial
application” in my plant?
A: Your current suppliers of measurement and control
should be able to discuss “Field Trial” opportunities, and
test installations of fieldbus technology that are most
suited to your specific industry, product, and plant
application. The Fieldbus Foundation Catalog could
also assist you in finding appropriate fieldbus device
suppliers for your application.
simulating and testing fieldbus signals and devices. In
addition, suppliers of hand-held communications
network test equipment are currently developing fieldbus
communications test devices that can be used to verify
the quality of potential or operating fieldbus segments
(Please refer to the Fieldbus Foundation Catalog).
Q: Can I add new wire, and mix it with installed wire
in the same fieldbus segment?
A: YES! The suppliers of fieldbus cable will be able to
provide assistance in the form of data sheets, training,
and services.
Q: How many devices can I put on one segment?
A: It depends on several factors. The maximum
number is 32 devices on one segment. Depending on
individual device power requirements (consult the device
manufacturer, as each device may have different power
requirements), a maximum of six (6) I.S. bus powered
devices can be on the same segment. These are worst
case numbers for maximum length. (Add information
based on curves from IS note (type A cable).)
27
© 1996 Fieldbus Foundation, Austin, Texas. All rights reserved.
Questions & Answers
Q: Can FOUNDATION fieldbus be redundant?
A: The Physical Layer supports bus redundancy. And,
as with analog or smart I/O and devices, there are a
number of other possibilities. Multiple devices used to
take the same measurement could operate on the same
or different fieldbus segments, and use different I/O card
cages and power supplies depending on the level of
redundancy desired. The initial H1 implementations do
not support redundancy, but the H2 implementations are
expected to do so.
Q: I need a 2 mile long network. How do I handle
this requirement?
Q: What happens if a field device gets attached to
the wrong wire pair?
A: One of the major advantages of the fieldbus protocol
is that there is no requirement to “ring out” each wire
pair, and insure that each pair is connected to the
correct instrument. In the case of the same segment, all
devices are connected in parallel to the same pair of
wires (maybe via a junction box).
In the case of devices on different bus segments, the
fieldbus protocol will detect each new device as it is
connected to the bus.
Q: Can I use my conventional (non-fieldbus) power
A: The Physical Layer specification recommends a
maximum length of 1900 m (6234 ft.) for type A (fieldbus)
wire. If this is not sufficient, up to 4 repeaters can be
used to extend the total length to five times 1900 m
(6234 ft.), for a total of 9500 m (31,168 ft. or 5.9 miles).
Q: I need optical (or radio) capability. Does fieldbus
have that? (if not - when?)
supplies for fieldbus loops?
A: The IEC/ISA Physical Layer Standard specifies a
power supply with particular impedance characteristics.
Power supplies meeting these specifications can be
purchased, or an external impedance matching network
can be added to an existing power supply
Q: I don’t understand “termination”. What is it, why
A: The IEC/ISA Physical Layer Standard provides for
do I need it, and is placement critical?
both radio and fiber optic communications modes. It is
expected products will be made available at some time
in the future.
A: Termination is the addition of the appropriate
Q: What happens if the bus gets shorted?
A: A temporary short (across the bus, or bus to ground)
will temporarily disrupt communications on the whole
bus segment, just as it would for other analog or digital
communications. However, the fieldbus protocol is error
detecting and correcting, and will not cause erroneous
control or indication based on a temporary or permanent
short (or open). Measurement and control devices and
systems designed to be used with fieldbus are expected
to have fail safe ability at least as capable as current
state of the art devices and systems.
fieldbus “terminator” at (or near) the end of each fieldbus
segment. The “terminator” has the same characteristic
impedance as the bus, and is required to preserve the
integrity of the signal. It is especially important on long
segments, and placement is critical — one should be
connected to the bus as close as possible to each end
of the segment. Some devices (especially I/O and/or
fieldbus test/simulation PC cards) include the option of
using an internal terminator. It is important to know in
this case whether it is connected.
Q: What happens if the bus gets grounded on one
side (or at one end)?
A: A temporary short to ground may temporarily disrupt
the bus, but any errors will be detected and handled by
the fieldbus protocol. A permanent short to ground
could disrupt the bus until the ground is removed (just as
with present analog or digital communications).
28
© 1996 Fieldbus Foundation, Austin, Texas. All rights reserved.
Definitions
9.0 DEFINITIONS
The following include some of the definitions listed in
ISA-S50.02 Part 2, Section 6.0.
Balanced transmission line: A pair of wires carrying an
AC analog or digital signal which is applied and
preserved differentially, with both sides isolated from
ground. Both ends must be terminated.
Intrinsic Safety Barrier: Physical entity that limits
current and voltage into a hazardous area in order to
satisfy Intrinsic Safety requirements.
Isolation: Physical and electrical arrangement of the
parts of a signal transmission system to prevent
electrical interference currents within or between the
parts.
NOTE: This definition is taken from IEEE Std 100-1984.
Bus: The trunk and all devices connected to it.
Bus powered device: Device that receives its operating
power via the fieldbus signal conductors.
Connector: Optional coupling device employed to
connect the wire medium to a fieldbus device or to
another section of wire (e.g. at a junction box).
Coupler: Physical interface between trunk and spur or
trunk and device.
Current mode: The serial, inductively coupled
communications mode of the Physical Layer described
in Clause 13 of the IEC/ISA Physical Layer Standard.
Galvanic isolator: A special device used to insure that
the circuit using one will not provide any current flow if
shorted to ground or power (within design limits).
H1: The 31.25 kbit/s fieldbus communications speed.
H2: One of two possible “High Speed” fieldbus
communications speeds. Can be either 1.0 Mbit/s or 2.5
Mbit/s.
Intrinsic Safety: Design methodology for a circuit or an
assembly of circuits in which any spark or thermal effect
produced under normal operating and specified fault
conditions is not capable under prescribed test
conditions of causing ignition of a given explosive
atmosphere.
Low Power Signaling: The communications mode
(addressed by this manual and the IEC/ISA Physical
Layer Standard, Part 2), which complies with the
proposed Clause 22 change to the MAU specification in essence, active devices optionally draw less current
when not in a “transmitting” mode, thereby allowing
more bus-powered devices without increasing the
power available to the bus. This is especially helpful for
an I.S. segment (as the total power in the I.S. area is
limited), or for extremely long segments, where the ability
to operate devices at the “end” of the wire could be
limited by the power supply and wire resistance.
MAU: The “Media Attachment Unit”, or interface
circuitry, between the bus and the rest of the
communications circuitry in a fieldbus device. This
circuitry is responsible for the communications of signals
to and from the bus, and for drawing the required
operating current for the fieldbus device from the bus
wires (in a bus powered device).
Medium: Cable, optical fiber, or other means by which
communication signals are transmitted between two or
more points.
Network: All of the media, connectors, and associated
communication elements by which a given set of
communicating devices are interconnected.
NOTE: This definition is taken from EN 50 020 :1977
(Intrinsically Safe Electrical Apparatus).
29
© 1996 Fieldbus Foundation, Austin, Texas. All rights reserved.
Definitions
Repeater: Device used to extend the range over which
signals can be correctly transmitted and received for a
given medium.
Topology: The shape of the network; Tree, Daisy Chain,
etc.
Segment: The section of a fieldbus that is terminated
in its characteristic impedance (by a “terminator”).
Segments can be linked by repeaters to form a complete
fieldbus network.
Transmission Line: A pair of wires having a specified
characteristic impedance, which is used to conduct an
AC (sometimes digital) signal between two points.
Terminators may be required at each end of the line to
prevent distortion and signal errors.
Separately powered device: Device which has power
wiring separate from its fieldbus wiring.
Transmitter: An active fieldbus device, containing
circuitry which applies a digital signal on the bus.
Shield: Surrounding earthed metallic layer to confine the
electric field within the cable and to protect the cable
from external electrical influence.
Trunk: Main communication highway acting as a source
of main supply to a number of other lines (spurs).
NOTE: Metallic sheaths, armors, and earthed concentric
conductors may also serve as a shield.
Spur: Branch line (i.e., a link connected to a larger one at
a point on its route) that is a final circuit.
NOTE: This definition is taken from IEEE Std 100-1984.
Standard Power Signaling: The communications mode
described in par 3.3 of this manual and in the IEC/ISA
Physical Layer Standard, Part 2, which complies with the
Clause 11 MAU specification - in essence, active
devices draw the same current whether in a
“transmitting” or “non-transmitting” mode. This is also
addressed in Clause 22.
NOTE: This definition is taken from IEEE Std 100-1984 modified.
Voltage mode: The communications mode addressed
by this manual (see par. 2.0), and of the IEC/ISA Physical
Layer Standard, Clause 2. The fieldbus signal is applied
and detected using parallel, voltage sensing.
1 Mbit/s: The lower of the two “high speed” (or “H2”)
fieldbus communications speeds.
2.5 Mbit/s: The higher of the two “high speed” (or “H2”)
fieldbus communications speeds.
31.25 kbit/s: The low speed (or “H1”) fieldbus
communications speed.
Terminator: A matching network used at, or near, the
end of a transmission line, which has the same
characteristic impedance of the line. It is used to
minimize signal distortion in the form of “reflected
waves” which can cause data errors.
30
© 1996 Fieldbus Foundation, Austin, Texas. All rights reserved.
Troubleshooting
10.0 TROUBLESHOOTING
10.3 Addressing and Polarity
Incorrect addressing makes a device appear dead.
It’s like saying, “Hello Charles,” to someone named
Fred. Make sure you have ruled this out before you
begin looking for installation problems.
Here are some guidelines on what to do if a fieldbus
network isn’t working.
“Isn’t working,” in the context of this document, means
that the network is either dead (no apparent
communication) or slow (too many communication
retries). Or one or more devices on an otherwise
working network appear dead or slow. These conditions
may be caused by incorrect installation, incorrect setup
(incorrect commissioning), or malfunctioning devices.
Other problems, such as a device transmitting bogus
data (with correct fieldbus protocol), are not Fieldbus
Physical Layer problems and are outside the scope of
this document.
A polarized device connected backwards may also
appear dead.
10.4 Works On Bench But Not in Network
If a field device works on the test bench but not in the
network, this can be due to addressing. If this device
has the same address as another device already on
the network, then it will appear to work when removed
from the network and to fail when reconnected.
10.1 Fieldbus Equipment with Built-in
Diagnostics
Another possibility is that the device has
malfunctioned and is causing heavy loading of the
network. Other devices may or may not communicate
with this device connected. The loading is not
apparent when the device is by itself but causes
excessive loading when added to an already loaded
network. This can be examined using an oscilloscope.
The scope must be a differential type (or battery
operated) and must be set to AC coupling to view the
wave packets (one packet = one message) traveling
on the network. If you see that the packets become
greatly attenuated when the offending device is
connected, then suspect that it has failed.
You may want to insist that your fieldbus field devices
have at least rudimentary diagnostic ability; such as
maintaining and divulging a count of erroneous
messages they’ve received during a given period.
A master with proper software can then determine
whether one or more devices are having a problem
communicating. This can be helpful in tracking down
a problem.
If, for example, a device indicates that 10 out of
every 100 messages it receives have errors (and are
consequently discarded), the signal at that device
should probably be examined to see whether it is too
small or is distorted.
10.5 Open Trunk
If the trunk has become open at some point, either
through accident or careless maintenance; the
network may appear to work some of the time and
not at other times, depending on whether devices try
to communicate across the break. Master software
will probably be capable of quickly providing a list of
devices that are “there” versus those that have
disappeared. This narrows down the location of the
break.
10.2 Reduction to Known Working State
Although it may be obvious, a powerful technique is
to reduce the size of the network until you get down
to something that works. At the junction box, for
example, you can disconnect major sections of the
network. When you’re satisfied that what is left works,
then add the sections back one at a time to try to
locate the offending section. Then begin picking the
offending section apart just as you did the whole
network.
Other evidence of a break can be gathered with an
oscilloscope. (The scope must be of a type and must
be set up as described earlier.) A break means that
only one terminator will be present, which will usually
cause almost double the normal signal level.
Remember that connecting and disconnecting
things from the live network may cause momentary
communication problems. Don’t confuse these with
the network problem you’re trying to find.
This situation will exist at either side of the break.
31
© 1996 Fieldbus Foundation, Austin, Texas. All rights reserved.
Troubleshooting
10.6 Handheld Master Works At One End of
Trunk But Not Other End
10.8 Test Equipment
A network analyzer may be useful. It performs
somewhat the same functions as a master but may
have more diagnostic capability. For example, it
should be able to quickly tell you if devices have been
addressed but didn’t answer. The analyzer is itself a
fieldbus device and must be specifically designed for
fieldbus.
Suppose that a master connected at end A of the
trunk talks to devices also connected at end A, but the
same master connected at end B of the trunk can’t
talk to devices connected at end B. This may indicate
that the network is shorted or shorted to ground or too
heavily loaded at the non-working end (end B). Or it
may mean that one of the trunk conductors is
continuous throughout the trunk, but the other is
broken with one side of the break open and the other
side of the break shorted to the other trunk conductor.
An oscilloscope is useful for observing the “quality” of
the signal packets. Various specific uses of the
oscilloscope were given earlier. The scope should be
differential or should be battery operated to avoid
grounding one side of the network through a scope
probe. Use probes that have at least 1 Megohm input
resistance and less than 1000 pf input capacitance.
10.7 Working Network Suddenly Stops
Suppose that there seems to be no communication
on a network that was previously OK. This could
mean that a device has malfunctioned and is shortcircuiting the network. Or it could mean that a
device has malfunctioned and is jabbering
(transmitting continuously and not allowing any other
communication). Both of these conditions may be
observable with an oscilloscope. (The scope must
be of a type and must be set up as described
earlier.) A short-circuit will often not be a perfect
short, so that highly attenuated packets may still be
seen. Jabbering will appear to be one long packet
with no observed breaks.
A DVM is useful for checking connections. The
ohmmeter function can tell you whether the network
has become shorted or whether one side is shorted to
ground. Before making such measurements, however,
you may have to disconnect the power supply in a
network powered fieldbus. Shutting off the power isn’t
enough. The reason is that the power supply may
contain DC paths that, although ineffective at
communication frequencies, have all 3 conductors (the
two network lines and ground) connected. There may
be devices other than the power supply that also
create this deceptive situation. Consult the
manufacturer’s specifications or temporarily
disconnect a suspect device before using the
ohmmeter. The DVM is also useful in checking supply
voltage in powered networks. The DVM input
capacitance should be less than 1000 pf.
If a device has short-circuited or is causing excessive
loading to the extent that it prevents or slows all
network traffic, it may not be easy to find. If it is
network powered it may present a short-circuit at
communication frequencies but not at DC. It may be
necessary to remove devices one by one to find the
offender.
32
© 1996 Fieldbus Foundation, Austin, Texas. All rights reserved.
References
11.0 REFERENCES
11.2 Organizations
Fieldbus Foundation
9390 Research Boulevard, Suite II-250
Austin, Texas 78795 USA
Tel: 1-512-794-8890
Fax: 1-512-794-8893
11.1 Documents
IEC 1158-2: 1993, Fieldbus Standard for Use in
Industrial Control Systems - Part 2: Physical Layer
Specification and Service Definition.
The International Society for
Measurement and Control
(formerly Instrument Society of America (ISA)
P.O. Box 12277
Research Triangle Park, NC, 27709 USA
Tel: 1-919-549-8411
Fax: 1-919-549-8288
ISA-S50.02-1992, Fieldbus Standard for Use in
Industrial Control Systems - Part 2: Physical Layer
Specification and Service Definition
dS50.02, Part 2 [Draft Standard] 1995, Fieldbus
Standard for Use in Industrial Control Systems - Part
2: Physical Layer Specification and Service Definition,
Amendment to Clause 22 (Formerly Clause 11 and
Clause 24) This document has been re-released in
1996 as Draft 2.
International Electrotechnical Commission (IEC)
3 Rue deVarembe
CH-1211 Geneve 20
Switzerland (Suisse)
Tel: 011-41-22-734-0150
Fax: 011-41-22-733-3843
Fieldbus Foundation Application Guide on Intrinsic
Safety, Revision 1.1, 21 September 1995, AG-163
ISA-RP12.6, “Wiring Practices for Hazardous
(Classified) Locations Instrumentation Part I: Intrinsic
Safety, Recommended Practice”
Fieldbus Foundation Physical Layer Profile
Specification, FF-816, Rev. 1.
NOTE: Further references are available in the listed
documents.
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© 1996 Fieldbus Foundation, Austin, Texas. All rights reserved.